1. Field of the Invention
The invention relates in general to the apparatus and method for estimating angle-of-arrival, and more particularly to the apparatus and method for estimating angle-of-arrival with suppressing multipath effects in a spread spectrum communication system.
2. Description of the Related Art
The wireless communication system is popularly used for convenience. For the sake of mobile callers' safety, the wireless communication system is recently required to be capable of determining the positions of mobile stations. With the positions of mobile callers, the emergency responsiveness of the emergency service, such as 911 service, can be improved. A network-based positioning system, for example, uses the given position of a base station (BS) to locate a mobile station (MS). The MS may be a cellular phone or other mobile communication device. The conventional positioning system determines the position of an MS by either the angle-of-arrival (AOA) of the incoming wireless signal or the timing-advance variable thereof.
FIG. 1 is a diagram showing a mobile positioning system by using AOA. In FIG. 1, an MS is positioned at the intersection of the first line of bearing (LOB) L1 and the second LOB L2. The first base station, BS1, and the second base station, BS2, each have an antenna array. A wireless signal 110 is emitted by the MS in all directions and then arrives at the base stations BS1 and BS2. The AOA θ1 of the BS1 and the AOA θ2 of the BS2 are obtained by performing an array signal processing on the received wireless signal. LOB L1 and LOB L2 are generated according to the AOAs θ1 and θ2, and then the MS is located at the intersection of LOB L1 and LOB L2. From the above description, at least two BSs are required to determine the position of the MS. For more accurate position of the MS, more BSs are required to estimate the AOA of the MS. However, the complexity of the base stations increases. In the existing wireless communication system, each base station using space diversity technique has two or three antennas only, while the above-mentioned base station with AOA positioning capability needs an antenna array. The base station using space diversity technique is widely used because it is simpler and able to reduce the fading effect effectively. Thus, great efforts in modifying the existing wireless system makes the AOA mobile positioning system difficult to realize in practice. In addition, the cost is much higher if antenna-array base stations are used.
There is also a to-be-solved issue, multipath. Multipath is the propagation phenomenon that results in radio signals' reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from terrestrial objects. The terrestrial objects can be mountains and buildings, for example. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. Therefore, the angle-of-arrival is mistakenly determined. Conventionally, the effects of multipath can be effectively eliminated by spread spectrum (SS). In spread spectrum communication system, such as CDMA (Code Division Multiplexing Access) system, a mobile station requests a one and only one pseudo-random code (PN), called spreading code, from a base station for transmitting data. Each mobile station spreads the data according to a unique spreading code to generate a spread signal for transmission. Also, the base station despreads the received spread signal from the mobile station according to the correspondent spreading code. The transmitted spread signal in the spread spectrum communication is the multiplication of the spreading code with the data. The spectrum of the spread signal is wider than that of the data because the spectrum of the spreading code is wider than that of the data. The noises attached to the spread signal can be effectively eliminated while despreading. For example, the spread code is the multiplication of each data bit with a sequence of 256 code bits, called chips. While despreading, the received spread signal is also multiplied by the same sequence of 256 code bits, and the noise is also reduced to 1/256 at best. However, the multipath still effects the determination of the angle-of-arrival even though the noise has been reduced by the spread spectrum.
The disadvantages of the conventional positioning approach is as following:                1. The base stations require antenna array in the AOA method, which is expensive and complicated compared to the popular base stations using space diversity technique.        2. The effects of multipath reduce the accuracy of the angle-of-arrival.        